Conditioning grinder

ABSTRACT

A travelling carriage carrying a motor driven grinding wheel at the end of a pivoted arm reciprocates back and forth along a steel slab workpiece. A hydraulic cylinder forces the wheel down against the workpiece, this downward movement being controlled in accordance with the power being supplied to the motor and the speed of reciprocating travel of the carriage. Control of the contact pressure of the wheel is thus based on the power being absorbed by the motor, which ensures a substantially constant rate of work output by the wheel. These conditions prevail during carriage travel at normal speed. When the carriage slows down to change direction at the ends of its reciprocal travel, a mechanism sensing the carriage speed reduces the rate of work output of the grinding wheel to obtain a more constant value for the work output per unit distance of travel, the result being more uniformity in the depth of cut. Provision is made for indexing the arm transversely of the workpiece at each end of carriage travel. The maximum magnitude of the acceleration and deceleration of the carriage at the travel ends is limited in order to increase the comfort of an operator seated on the carriage. The grinding wheel peripheral speed is kept constant by a radioactive source and detector system that measures wheel diameter and speeds up the motor as the wheel wears. To facilitate this speed control a D.C. motor is used. As an alternative to a moving carriage, the structure carrying the grinding wheel can be stationary, the workpiece holder then reciprocating, the same controls over the grinding wheel pressure being exercised.

United States Patent McDowell et al.

[4 1 June 6,1972

[54] CONDITIONING GRINDER [72] Inventors: Philip R. McDowell; Charles W.Vekovious, both of Downsview, Ontario, Canada [73] Assignee: G & BAutomated Equipment Limited,

Downsview, Ontario, Canada 22 Filed: Feb. 16,1971

21 Appl.No.: 115,364

[52] U.S.Cl ..51/35,5l/93,5l/l65.77 [51] Int. Cl ..B24b 7/02 [58] FieldofSearch ..5l/35,93, 165 R, 165.77

3,113,405 12/1963 Schneider .....51/l65 R 2,726,490 12/1955 Lowe5l/l65.77 X

3,089,287 5/1963 Dilks ..5l/35 Primary Examiner-Harold D. WhiteheadAttorney-Peter Kirby, Charles P. Curphey and Norris M. Eades 57 ABSTRACTA travelling carriage carrying a motor driven grinding wheel at the endof a pivoted arm reciprocates back and forth along a steel slabworkpiece. A hydraulic cylinder forces the wheel down against theworkpiece, this downward movement being controlled in accordance withthe power being supplied to the motor and the speed of reciprocatingtravel of the carriage. Control of the contact pressure of the wheel isthus based on the power being absorbed by the motor, which ensures asubstantially constant rate of work output by the wheel. Theseconditions prevail during carriage travel at normal speed. When thecarriage slows down to change direction at the ends of its reciprocaltravel, a mechanism sensing the carriage speed reduces the rate of workoutput of the grinding wheel to obtain a more constant value for thework output per unit distance of travel, the result being moreuniformity in the depth of cut. Provision is made for indexing the armtransversely of the workpiece at each end of carriage travel. Themaximum magnitude of the acceleration and deceleration of the carriageat the travel ends is limited in order to increase the comfort of anoperator seated on the carriage. The grinding wheel peripheral speed iskept constant by a radioactive source and detector system that measureswheel diameter and speeds up the motor as the wheel wears. To facilitatethis speed control a DC. motor is used. As an alternative to a movingcarriage, the structure carrying the grinding wheel can be stationary,the workpiece holder then reciprocating, the same controls over thegrinding wheel pressure being exercised.

8 Claims, 16 Drawing Figures PATENTED H 61m SHEET 2 0F 9 PATENTEUJUN61972 3,667,165

SHEET 30F 9 CONDITIONING GRINDER This invention relates to improvementsin a conditioning grinder, i.e., a grinding machine for use in a mill,such as a steel mill, for removing scale and/or other imperfections fromthe surface of a billet, slab or other workpiece.

The object of the present invention is to provide a conditioning grinderthat will remove from the workpiece a surface layer the thickness ofwhich can be more accurately controlled so as to be more uniformthroughout the surface of the workpiece, than has been possible withprevious conditioning grinders. For example, it may be desired to removea surface layer of a thickness of one thirty-second of an inch.Circumstances will vary and it may at times be required that a thickeror a thinner layer be removed. However, removal of more metal than isstrictly required represents significant wastage. This is especiallytrue when the workpiece is a slab of one of the expensive specialtysteels, such as stainless steel or a tool steel.

One of the difficulties with prior conditioning grinders has been that,in order to ensure at least the required depth of cut throughout theentire surface of the workpiece, it has been necessary to accept thedisadvantage of taking an excessive depth of cut in parts of thesurface. In other words, control of the depth of cut over the variousparts of the surface has been relatively crude.

The principal object of the present invention is to provide a machine inwhich this control is improved, with the obvious advantage ofminimization of wastage of workpiece material.

One of the specific difficulties experienced with prior grinders hasbeen a tendency for the grinding wheel to remove an excessive amount ofmetal near the ends of the workpiece when the relative travel betweenthe wheel and the workpiece slows down. It is conventional inconditioning grinders for the grinding wheel to be reciprocated back andforth along the work. For this purpose the workpiece holder can bereciprocated beneath a stationary assembly containing the grinding wheeland its drive mechanism, or the workpiece can be held in a stationaryholder with the grinding wheel mounted on a reciprocating carriage.There will also be provision for indexing the grinding wheel across theworkpiece on successive longitudinal passes of the wheel in order totake a similar cut from each portion of the workpiece surface.

When the workpiece is massive, it is convenient to adopt the alternativementioned above, namely to use a fixed workpiece with a reciprocatingcarriage carrying the grinding wheel. It will be desirable toreciprocate the grinding wheel accurately between the two ends of theworkpiece, i.e., not to miss each extreme end portion, or, on the otherhand, to pass beyond either of these ends. The wheel will be urgedagainst the workpiece with substantial pressure, so that it would tendto drop off" the workpiece, if taken beyond the end. There would then berisk of damage, if the direction of travel were immediately reversed.Difficulty in maintaining automatically the necessary control for suchan accurate registration of the grinding wheel and the workpiece ends,has been experienced in prior grinders, and it is a further object ofthe present invention to provide improvements in this regard.

It is also known in grinding machines to make provision for sensing thewear of the wheel and for maintaining its peripheral linear speedsubstantially constant by increasing the motor speed as the wheel wears.Such arrangements have not proved entirely satisfactory in operation,however, and it is a still further object of the present invention toprovide improvements in regard to the reliability of such mechanisms forsensing wheel wear.

Yet another object of the present invention is to provide a conditioninggrinder in which the operations are carried out in a highly automatedmanner, thus speeding production and minimizing the results of operatorerrors.

Other features and advantages of the machine will be apparent from thespecific description below.

One form of grinder constructed in accordance with the various featuresof the present invention is illustrated in the accompanying drawings. Itis to be understood that this machine is illustrated by way of exampleonly, and not by way of limitation of the broad scope of the invention,which broad scope is defined in the appended claims.

In the drawings:

FIG. 1 is an end view of the machine, with various casing and otherparts cut away for clarity of illustration;

FIG. 2 is a plan view of the machine seen in FIG. 1, again with certainparts cut away;

FIG. 3 is a side view of the machine, as seen from the right hand sideof FIG. 1;

FIG. 4 is an enlarged'view of the grinding wheel am, also with certainparts cut away to show interior structure;

FIG. 5 is a section on the line V-V in FIG. 4;

FIG. 5a is a section on the line Va--Va in FIG. 4;

FIG. 6 is a further fragmentary view showing details of the grindingwheel assembly; 7

FIG. 7 is a section on the line VII-VII in FIG. 6;

FIG. 8 is an elevation view of the carriage drive assembly with thecable omitted;

FIG. 9 is a plan view of FIG. 8;

FIG. 10 is a view taken on the line X-X in FIG. 8;

FIG. 1 l is a fragmentary view showing a guide roll;

FIG. 12 is a section on the line XII-XII in FIG. 11, except that theparts are in a different position;

FIG. 13 is a small scale plan view illustrating an alternativearrangement;

FIG. 14 is a diagramshowing the manner of actuation of an indexingcylinder; and

FIG. 15 is a general schematic circuit diagram.

GENERAL CONSTRUCTION Firstly with reference to FIGS. 1 to 3, it will benoted that the machine comprises a main structure in the form of a maincarriage 10 having a main assembly 1 1 connected to a remote assembly 12by means of a pair of overhead beams 13. The asv semblies 11 and 12 andthe beams 13 together define an archway extending over a space in whicha workpiece slab 14 is shown located on a fixed table 15. The carriage10 is mounted on flanged wheels 16 running on fixed rails 17.

Extending across the archway is a pair of fixed rods 20 on whichrespective sleeves 21 are slidably mounted. The sleeves 21 serve tosupport between them a secondary carriage 22 from which a DC. electricmotor 23 is suspended. The shaft of the motor 23 drives a pulley 24 fromwhich a belt 25 drives to a further pulley 26 that is mounted on a shaft27 of a grinding wheel 28.

GRINDING WHEEL ARM As best seen from FIGS. 4 and'S, the shaft 27 of thegrinding wheel 28 is joumalled in a grinding wheel assembly 30 that ismounted on the end of a grinding wheel arm 31 pivotally supported by apin 32 from the secondary carriage 22.

Control of pivotal movement of the arm 31 about the pin 32 is exercisedby a hydraulic cylinder 33 one end of which is pivotally mounted on asleeve 21 by a bracket 34. The piston 35 of this cylinder 33 is securedby a pivotal bracket assembly 36 to a main outer member 37 of the arm31.

Rotatably mounted within an open end of the arm member 37 is an innersleeve 38 secured to a plate 39 having a projection 40 connected to thepiston 41 of a further hydraulic cylinder 42 mounted on the arm member37. This cylinder effects rotation of the plate 39 through a limitedangle each side of the central position shown in FIG. 5a. This rotationis transmitted through splines 43 on an inner arm member 44, the outerend of which supports the grinding wheel assembly 30. The operator isthus able to rock the grinding wheel through a limited angle to takecare of any special workpiece conditions. In normal automatic operationthe wheel will be centered, i.e., with the axis of the shaft 27horizontal.

Limited longitudinal movement of the inner arm member 44 relative to themain arm member 37 is achieved by the admission of hydraulic fluideither to a space 45 to act on an end surface 46 of the inner arm member44 to force the same outwards, or to an annular space 47 to act on arear surface 48 of the arm member 44 to retract the same. Thislongitudinal travel of the grinding wheel assembly 30 serves the purposeof relaxing or applying tension in the belt 25, as required formaintainance or to take up any stretching of the belt.

GRINDING WHEEL ASSEMBLY More details of the grinding wheel assembly 30are shown in FIGS. 6 and 7. This assembly includes a casing 50 coveringthe portion of the grinding wheel 28 remote from that which projectsinto contact with the workpiece. In opposite sides of .the casing 50 isa pair of aligned slots 51 extending radiallyin relation to the shaft27. A radioactive source 52, of Krypton, for example, and a compatibleradioactive detector 53, e.g., a tungsten wire, are mounted respectivelyon arms 54 and 55 that are both secured to a pin 56 pivotally mounted inthe easing 50. Connected to the arm 55 is a gear tooth segment 57 thatmeshes with a worm wheel 58 driven by a motor 59. The purpose of thisarrangement is to detect wear in the diameter of the wheel 28. Radiationfrom the source 52 sensed by the detector 53 energizes the motor 59 torotate the assembly towards the radially inward position shown in FIG.6.

As soon as the radiation is obscured from the detector by the'edge ofthe grinding wheel 28, the motor 59 is de-energized. The parts remain inthe position acquired until subsequent wheel wear again reveals thesource to the detector. A potentiometer 60 mounted on the shaft of themotor 59 provides the necessary sensing of the extent to which themechanism has tracked inwardly, and hence an indication of the amount ofwear from the wheel 28. This information is employed to vary the fieldstrength and hence the speed of the motor 23 (see FIG. so as to maintainthe linear speed of the grinding wheel periphery substantially constant.

Use of a radioactive system increases the reliability of detection. Theassembly 30 is subjected to substantial vibration in service, and theair is thick with swarf, i.e., flying, hot fragments of metal andgrinding wheel material. Detection systems using light or ultrasonicbeams have proved unsatisfactorily reliable under these adverseconditions.

SWARF COLLECTOR Most of the swarf generated by the wheel is thrown intoa swarf collector 61 trough an aperture 62 in the assembly 12. Baffles63 tend to deposit the solid material in a tray that can be removedthrough a door 64. Air is sucked out through a conduit 65 which isconnected by suitable flexible piping to a source of suction and tofurther filtering and air cleaning apparatus (not shown).

OPERATOR's S CARRIAGE RECIPROCATION The main carriage 10 is reciprocatedalong the rails 17 by a capstan assembly shown mainly in FIGS. 3 and 8to 10. This assembly consists of an hydraulic capstan motor 70, theshaft of which drives a multi-groove sheave 71. A similar multigroovesheave 72 is mounted as an idler on a slightly inclined shaft 73 (seeFIG. 10). As shown in FIG. 3, a cable 74 extends under tension between apair of fixed anchors 75 and 76. At

each end of the machine the cable 74 enters by passing under a groovedidler pulley 77, details of which are shown in FIGS. 11 and 12. Thepulley 77 is urged downwardly by a spring 79 to press against the cable74. Should there be a loss of tension in the cable, one or both of thepulleys will be permitted to move downwardly as shown in FIG. 11, a pin78 moving down to release a switch 780 to signal a loss of cable tensionand stop the operation of the motor 70.

Entering the machine from the lefi hand side of FIG. 3 the cable 74,afier passing under the pulley 77, enters a first groove 71a of thesheave 71 and extends across to engage the top of a corresponding groove72a ofthe sheave 72. Because of the tilt of the sheave 72, when thecable leaves the groove 72a at the bottom of the sheave, it is nowaligned with the second groove 71b of the sheave 71. Since the sheave 71is not tilted the bottom and top locations of the groove 71b are alignedwith each other, so that the cable, after passing 180 around the sheave7l,emerges in alignment with the upper location of groove 72b. Thispassing of the cable 74 back and forth between the sheaves, with 180engagement at each turn, continues until finally the cable engagesmomentarily in the upper location of the fourth groove 72d of sheave 72finally to extend under the right hand pulley 77 to the anchor 76.

The cable tension is maintained by a piston 84 in a hydraulic cylinder85, the piston 84 acting to urge the sheave 72 away from the sheave 71.

This type v,of capstan drive is an improvement over those previouslyemployed in grinders, in which the power for effecting travel of thecarriage has been conveyed through the wheels engaging the rails. Thelatter method of driving is prone to slippage, with consequentinaccuracies both in the speed of travel and in the exact location ofthe carriage. To facilitate increased automation of the machine it isimportant to be able to reciprocate the carriage accurately between twolimit positions that correspond to the ends of the workpiece. This needin turn requires reliable information as to the location of the carriageat any instant, as well as the ability to control its speed,acceleration and deceleration without introduction of errors that mightarise from such extraneous causes as slippage caused by a patch of oilon a rail. The present form of capstan motor and drive mechanismprovides this reliability, the shaft 80 of the motor 70 and sheave 71being connected by chain gearing 81 to a shaft 82 of a potentiometer 83that provides an accurate electrically indication of the position of thecarriage at all times.

ALTERNATIVE OF RECIPROCATED WORKPIECE' I FIG. 13 illustrates analternative arrangement in which the main structure 10a of the grinderis fixed. A capstan motor 70 and sheave assembly, as before, drives acable 74a which passes around idler rollers 88 and 89 at each end of atrack 90, along which a workpiece holder 15a can be reciprocated bymeans of the cable 74a, the two ends of which are secured to saidholder. This arrangement provides essentially the same relativereciprocation between the grinding wheel and the workpiece in the firsthorizontal direction, i.e., the direction along the workpiece, as doesthe arrangement of FIGS. 1 to 12.

TRANSVERSE INDEXING OF GRINDING WHEEL Indexing of the grinding wheelrelative to the workpiece in the other horizontal directionperpendicular to said first direction takes place at the end of eachsuccessive longitu dinal reciprocation. This effect is achieved by meansof an indexing hydraulic cylinder (FIGS. 1 and 2) that is fixed to theremote assembly 12 at 101 and has a piston 102 connected at 103 to thesecondary carriage 22. The piston 102 is shown extended in the drawings.It will be apparent that withdrawal of the piston 102 will cause thecarriage 22 to slide on the rods 20 from right to left, moving thegrinding wheel arm 31 and hence the wheel 28 itself transversely acrossthe workpiece. The cylinder 100 is actuated as illustrateddiagrammatically in FIG. 14. A piston 104 in a measuring cylinder 105 isdriven to the right by a control cylinder 106 to draw oil into a chamber107 through a non-return valve shown diagrammatically at 108. The amountof oil drawn into the chamber 107 is limited by the travel of the pistonrod 109 which strikes a stop 110, the position of which is adjustable bya hand wheel 111. Reversal of the control cylinder 106 will cause themeasured amount of oil drawn into the chamber 107 to be forced out againthrough a non-retum valve 112 and into the cylinder 102, thus indexingthe secondary carriage 22 across the workpiece by the desired smallamount. The control cylinder 106 is actuated by an indexing control 113in accordance with the general circuit explained below. A manualsecondary carriage control 114 can exercise overriding control overmovement of the piston 102.

OPERATIONzGRlNDlNG WHEEL MECHANISM The general control circuits areshown diagrammatically in FIG. 15.

A wattmeter 120 is connected in the supply to the armature of the DC.motor 23, the field of which is energized through the potentiometer 60for the speed control purpose already explained. The wattmeter 120provides an analogue output in the form of a voltage that increaseslinearly with the power 7 being used by the motor 23. This voltage isfed to one input of a comparator 121, the output of which supplies theinput of a control amplifier 122. The comparator 121 receives a secondinput from an auto switch 123. Assuming this auto switch to be in theManual position shown, it receives a voltage from a potential divider124 which serves as the manual head control. The comparator 121 comparesits two inputs, the voltage received from the switch 123 representing adatum or predetermined level of power required to be expended by thegrinding motor 23, this datum being set manually by the divider 124. ifthe motor 23 is working harder than the set level, the comparator 121will energize the amplifier 122 to actuate a grinding arm displacementvalve 125 to raise the piston 35 of the cylinder 33 that controls thegrinding wheel arm. Raising of the grinding wheel to reduce its depth ofcut into the workpiece will reduce the load on the motor. Conversely,too light a load, if detected by the comparator, will energize the headvalve 125 to lower the grinding wheel arm.

When the operator moves the switch 123 over to Auto, the datum input tothe comparator 121 is supplied from the adjustable arm'of a depth of cutpotential divider 126 that will have been already adjusted to thedesired setting. This datum level will remain constant so long as thediver 126 continues to be energized with a steady potential from acarriage speed sensor 127, e.g., a tachometer connected to the carriagemotor 70. Reduction of the speed of the carriage at each end of eachreciprocation, when changing direction, will, however, cause acorresponding reduction in the datum level, with a consequent reductionin the downward displacement of the grinding wheel arm and hence in therate of work output by the grinding wheel motor required to balance thecomparator. Ideally, this rate of work output, i.e., grinding work perunit time, should be reduced approximately linearly with speed toproduce instead a substantially constant work output per unit distanceof travel. This should result in an even depth of out being taken alongthe full length of the workpiece, thus avoiding the excess depth of cutthat has been experienced in prior art machines at the ends when thespeed of travel has had to be reduced to change direction.

In practice, whether a closely constant work output per unit distance oftravel is achieved, or not, will depend largely on the tolerances builtinto the machine. It is therefore desired to stress that the presentinvention is not restricted to machines that exactly achieve thisdesirable end. The essential feature of the present invention resides inthe fact that, by virtue of making the grinding arm displacement controldependent on the carriage speed, as well as on the motor load, anadditional facility has been added. The prime utility of such additionalfacility is the ability to achieve a greater uniformity in the grindingdepth of cut taken throughout the surface of the workpiece.

An example of a respect in which the grinding arm displacement may notbe varied exactly linearly with carriage speed arises at the instant thecarriage stops to reverse direction, i.e., when the speed is zero. Toavoid the grinding wheel being raised completely from the workpiecesurface at this moment, the output of the carriage speed sensor can bearranged never to fall below a small, minimum level. Similarly,provision may be made in the choice of the circuit constants to allowfor windage and friction losses in the motor and drive, i.e., thedifference between the power measured by the wattmeter and that actuallydelivered at the grinding wheel surface.

A minimum load sensor 130 receives the output from the wattmeter 120through contacts 131 of the auto switch, which are closed when in theAuto mode. If this output reaches a certain low value indicative ofvirtually no work output at the wheel,as may occur if the grinding wheelhas accidentally passed beyond the end of the workpiece, the sensor 130overrides the comparator 121 to cause the amplifier 122 to raise thegrinding arm. The operator must then take over and correct the situationin temporary Manual operation.

OPERATlONzCARRlAGE The potentiometer 83 already described constitutes acarriage position sensor. Its output feeds to a pair of comparators and141 that also receives signals from respective right and left adjustablelimit devices 142 and 143. To set up the operation, the operator willtrack the carriage in manual control to each limit position required.These limit positions will normally be when the grinding wheel is justat each end of the workpiece. He will then adjust the respective limitdevice to produce balance in its associated comparator, as indicated bya meter incorporated therein. Each comparator 140, 141 will include abias so that the comparator will yield a characteristic output signal oneach subsequent occasion when the carriage approaches within a selecteddistance of the limit position set by the operator, this selecteddistance being that required for smooth deceleration of the carriage.

The outputs from the comparators 140, 141 energize an auto carriagecontrol 144 which, when a carriage auto switch 145 is in the Auto mode,controls a rotary hydraulic pump 147 driven from an AC. squirrel-cageinduction motor 146. The pump 147 powers the carriage motor 70. Thevolume of oil available from the pump 147 is thus externally adjustableand from the initiation of deceleration by the control 144, thisexternal adjustment is stroked from the full oil volume coming from oneport of the pump, through zero, to the same volume now coming from theopposite port of the pump. This stroking arrangement allows variablevolume producing speeds from zero to maximum in either direction. Therate that the pump volume is thus externally varied will control therate of acceleration or deceleration. During deceleration, the carriagemotor 70 acts as a prime mover in an attempt to run the electrical motor146 over its synchronous speed, as determined by the line frequency andits number of poles. Once this occurs, the motor 146 acts as aninduction generator and regenerates the carriage energy into the ACline. A valve 149 is provided for use basically as a shut-off valve anda pump by-pass valve for emergency stop conditions. As soon as thecarriage stops, a signal from the carriage speed sensor 127 is receivedin a carriage stop sensor 148 which energizes the carriage control 144to reverse the direction of oil flow from the pump 147 to accelerate thecarriage motor 70 smoothly (again with a maximum value of acceleration)up to full carriage speed in the opposite direction.

The sensor 148 also simultaneously actuates the indexing control 113(FIG. 14) which moves the piston 104 to the left to cause the piston 102to index the grinding wheel arm transversely of the workpiece. Theindexing control 1 13 immediately returns the piston 104 as far as itwill go to the right to await a subsequent actuation, which will occurwhen the carriage stops momentarily at the other end of its travel. Whenthe carriage auto switch is in the Manual mode, control of the carriageis exercised by conventional circuits in a manual carriage control 150.

We claim:

1. A conditioning grinder comprising a. a main structure,

b. a workpiece holder,

c. an arm movably mounted on said main structure,

d. a grinding wheel mounted on said arm,

e. displacement means acting between said structure and said arm to movethe wheel in a displacement direction into and out of grinding contactwith a surface of a workpiece mounted on said holder,

f. an electric motor connected to the grinding wheel for driving thesame,

g. means for effecting relative movement between said structure and theworkpiece holder in a first direction of extent of the workpiece surfaceto travel along the workpiece surface, said first direction beingsubstantially perpendicular to said displacement direction,

h. first sensing means for sensing the power absorbed by said motor,

i. second sensing means for sensing the speed of said travel,

j, and control means connected to said first and second sensing meansand to said displacement means for moving the grinding wheel towards theworkpiece surface in response to reduction of said power below apredetermined level and for moving the grinding wheel away from saidworkpiece surface in response to increase of said power above saidpredetermined level, and for reducing said predetermined level uponreduction of said speed whereby to render the work output of thegrinding wheel per unit distance of travel more nearly constant.

2. A grinder according to claim 1, wherein said means for effectingrelative movement between the structure and the workpiece holdercomprises k. means for effecting reciprocation of the grinding wheelback and forth along the workpiece between a pair of extreme positionscorresponding to ends of the workpiece.

3. A grinder according to claim 2, wherein said main structure includesl. a secondary structure on which said arm is movably mounted and m.means for moving said secondary structure stepwise relative to theremainder of the main structure in a second direction of extent of theworkpiece surface, said second direction being substantiallyperpendicular both to said first direction and to said displacementdirection whereby to index the grinding wheel across the workpiece uponsuccessive reciprocations thereof.

4. A grinder according to claim 2, wherein the workpiece holder isstationary and the main structure comprises a carriage including wheelsfor engaging fixed rails, the grinder further including l. a cableextending between fixed points at opposite ends of said rails,

m. a capstan motor mounted on said carriage,

n. and sheaves drivingly engaging the cable and driven by said capstanmotor for positive driving of said carriage.

5. A grinder according to claim 4, including,

0. means supplying power to said capstan motor for altemately drivingthe same in opposite directions for effecting said reciprocation of thegrinding wheel along the workpiece,

p. and means for limiting the magnitude of acceleration and decelerationof said capstan motor.

6. A grinder according to claim 2, wherein the main structure isstationary and the workpiece holder comprises a carriage includingwheels for engaging fixed rails, the grinder further including 1. acable having ends connected to respective ends of said carriage,

m. a capstan motor mounted on said structure,

11. and sheaves drivingly engaging the cable and driven by the capstanmotor for positive driving of said carriage.

7. A grinder according to claim 1, including k. third sensing meansmounted on said arm for sensing the diameter of the grinding wheel,

1. and further control means connected to said third sensing means andto the electric motor for increasing the rotational speed thereofinversely with said diameter whereby to maintain a substantiallyconstant linear speed at the periphery of said wheel.

8. A grinder according to claim 7, wherein said third sensing meanscomprise m. a radioactive source located on one side of the wheel,

n. a detector for energization by and aligned with said source on theother side of the wheel,

0. means for moving said source and detector radially of the wheel,

p. means connected to said detector for actuating said moving means tomove the source and detector radially inwardly until the source isobscured from the detector by the grinding wheel,

q. and means for transmitting the position of said moving means to saidfurther control means.

1. A conditioning grinder comprising a. a main structure, b. a workpieceholder, c. an arm movably mounted on said main structure, d. a grindingwheel mounted on said arm, e. displacement means acting between saidstructure and said arm to move the wheel in a displacement directioninto and out of grinding contact with a surface of a workpiece mountedon said holder, f. an electric motor connected to the grinding wheel fordriving the same, g. means for effecting relative movement between saidstructure and the workpiece holder in a first direction of extent of theworkpiece surface to travel along the workpiece surface, said firstdirection being substantially perpendicular to said displacementdirection, h. first sensing means for sensing the power absorbed by saidmotor, i. second sensing means for sensing the speed of said travel, j.and control means connected to said first and second sensing means andto said displacement mEans for moving the grinding wheel towards theworkpiece surface in response to reduction of said power below apredetermined level and for moving the grinding wheel away from saidworkpiece surface in response to increase of said power above saidpredetermined level, and for reducing said predetermined level uponreduction of said speed whereby to render the work output of thegrinding wheel per unit distance of travel more nearly constant.
 2. Agrinder according to claim 1, wherein said means for effecting relativemovement between the structure and the workpiece holder comprises k.means for effecting reciprocation of the grinding wheel back and forthalong the workpiece between a pair of extreme positions corresponding toends of the workpiece.
 3. A grinder according to claim 2, wherein saidmain structure includes l. a secondary structure on which said arm ismovably mounted and m. means for moving said secondary structurestepwise relative to the remainder of the main structure in a seconddirection of extent of the workpiece surface, said second directionbeing substantially perpendicular both to said first direction and tosaid displacement direction whereby to index the grinding wheel acrossthe workpiece upon successive reciprocations thereof.
 4. A grinderaccording to claim 2, wherein the workpiece holder is stationary and themain structure comprises a carriage including wheels for engaging fixedrails, the grinder further including l. a cable extending between fixedpoints at opposite ends of said rails, m. a capstan motor mounted onsaid carriage, n. and sheaves drivingly engaging the cable and driven bysaid capstan motor for positive driving of said carriage.
 5. A grinderaccording to claim 4, including, o. means supplying power to saidcapstan motor for alternately driving the same in opposite directionsfor effecting said reciprocation of the grinding wheel along theworkpiece, p. and means for limiting the magnitude of acceleration anddeceleration of said capstan motor.
 6. A grinder according to claim 2,wherein the main structure is stationary and the workpiece holdercomprises a carriage including wheels for engaging fixed rails, thegrinder further including l. a cable having ends connected to respectiveends of said carriage, m. a capstan motor mounted on said structure, n.and sheaves drivingly engaging the cable and driven by the capstan motorfor positive driving of said carriage.
 7. A grinder according to claim1, including k. third sensing means mounted on said arm for sensing thediameter of the grinding wheel, l. and further control means connectedto said third sensing means and to the electric motor for increasing therotational speed thereof inversely with said diameter whereby tomaintain a substantially constant linear speed at the periphery of saidwheel.
 8. A grinder according to claim 7, wherein said third sensingmeans comprise m. a radioactive source located on one side of the wheel,n. a detector for energization by and aligned with said source on theother side of the wheel, o. means for moving said source and detectorradially of the wheel, p. means connected to said detector for actuatingsaid moving means to move the source and detector radially inwardlyuntil the source is obscured from the detector by the grinding wheel, q.and means for transmitting the position of said moving means to saidfurther control means.